Sounds like your power supply is on its last legs. Astec was notorious for producing poor power supplies for Macs, as I am sure you are well aware of (I am surprised that an Astec supply would be in a IIfx; you would figure a Sony PS would have been used, considering the IIfx was the "flagship" Mac model for a short period of time).

There could also be potential problems with the motherboard. Look for bulging caps, and leakage.

100th post, btw.